This invention relates to a new process for the preparation of substantially closed-cell rigid foams containing urethane, urea, biuret, and isocyanurate groups and to their use as insulating materials and/or for reinforcing mechanical structural components.
Processes for the preparation of substantially closed-cell rigid foams containing urethane, urea, biuret, and isocyanurate groups are generally known. A review of the preparation of corresponding rigid foams can be found, for example in Kunststoff-Handbuch, Vol. VII, "Polyurethane", edited by R. Vieweg and A. Hochtlen, pages 504 et seq (Verlag Carl Hanser, Munich, 1966). Polyisocyanurate ("PIR") foams and modified PIR foams are described, for example, in Advances in Urethane Science and Technology, Vol. 3, pages 141 et seq. Vol. 2, pages 241 et seq (Technomic Publishing Co., Inc., 1974 and 1973).
To prepare such rigid foams, aromatic polyisocyanates are normally reacted with polyols of relatively high molecular weight, preferably polyether or polyester polyols, in the presence of blowing agents, catalysts, flameproofing agents, and other auxiliaries and additives. Corresponding mixtures of the polyether or polyester polyols can also be used. One of the determining factors in using mixtures is the reduction in costs that can often be attained by mixing expensive polyester polyols suitable for the production of high-quality rigid polyurethane foams with inexpensive polyether polyols. Suitable polyester polyols of low viscosity for the production of rigid polyurethane foams having good mechanical properties are described, for example, in German Offenlegungsschrift 2,704,196.
A particular advantage of using polyester polyols as the relatively high molecular weight polyhydroxyl compound in the production of rigid foams containing urethane and isocyanurate groups is an improvement in flame retardancy and a reduction in brittleness of the foams. A disadvantage, however, is that, on account of their relatively high polarity compared with polyether polyols (preferably polyoxypropylene polyols), polyester polyols are miscible only to a limited extent with the low-boiling halogenated hydrocarbons used as blowing agents. In particular, when using flameproofed high-index rigid isocyanurate-forming polyurethane foam formulations in which the polyol component consists predominantly of difunctional polyester polyols, adequate quantities of halogenated hydrocarbons blowing agents can longer be dissolved in the polyester polyol. According to German Offenlegungsschrift 3,642,666, this disadvantage is obviated by the concomitant use of polyester polyols containing alkyl side groups.
All the processes mentioned above require the use CFC blowing agents to obtain desirable rigid-foam characteristics such as, for example, strength, adhesion outer layers, and reduced inflammability. As can be seen from a comparison example described below, corresponding rigid loans show major disadvantages in the absence of CFC blowing agents, namely, brittleness, inflammability, and non-adhesion to outer layers.
Accordingly, the problem addressed by the present invention is to provide a new process for the preparation corresponding rigid foams having high strength, adhesion, and reduced inflammability which (a) uses conventional formulation constituents and yet (b) avoids the use of CFC blowing agents. It has now surprisingly been found that these problems are solved by the process according to the invention described in detail below.